Crystal oscillator

ABSTRACT

Provided is a crystal oscillator, including: a crystal; an oscillating circuit including a first oscillating transistor and a second oscillating transistor, where the first oscillating transistor and the second oscillating transistor are configured to provide transconductance for starting oscillation and maintaining oscillation of the crystal; a first driving circuit configured to generate a stable reference current; and a second driving circuit, configured to supply an operating voltage to the oscillating circuit and make an operating current of the first oscillating transistor and the second oscillating transistor be a stable current according to the reference current, where the operating voltage is used to control the first oscillating transistor and the second oscillating transistor to operate in a sub-threshold region.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of international applicationNo. PCT/CN2018/078507, filed on Mar. 9, 2018, which is herebyincorporated by reference in its entirety.

TECHNICAL FIELD

Embodiments of the present disclosure relate to the field of circuits,and in particular, to a crystal oscillator.

BACKGROUND

A Pierce oscillator (or referred to as a Pierce crystal oscillator) is acrystal oscillator with an inverter structure, and two metallic oxidesemiconductor field effect transistors (MOSFET) in the Pierce oscillatorcontribute transconductance, which is beneficial for reducing powerconsumption required to maintain oscillation. However, the Pierceoscillator has some defects: bias currents of two MOS transistors areaffected by gate-source voltages V_(GS)S of the MOS transistors and asum of the gate-source voltages of the two MOS transistors is equal to apower supply voltage V_(DD), therefore, if V_(DD) is large, V_(GS)S ofthe MOS transistors are necessarily large, which may lead to a serioushigh-order effect, and the problem is that although power consumption ofthe MOS transistors is large, the MOS transistors in fact do not providesufficient transconductance, which is not beneficial for fast startingoscillation of the crystal.

SUMMARY

An embodiment of the present disclosure provides a crystal oscillator,which could provide large transconductance for fast starting oscillationof a crystal with low power consumption.

In a first aspect, provided is a crystal oscillator including: acrystal; an oscillating circuit including a first oscillating transistorand a second oscillating transistor, where the first oscillatingtransistor and the second oscillating transistor are configured toprovide transconductance for starting oscillation and maintainingoscillation of the crystal; a first driving circuit configured togenerate a stable reference current; and a second driving circuit,configured to supply an operating voltage to the oscillating circuit andmake an operating current of the first oscillating transistor and thesecond oscillating transistor be a stable current according to thereference current, where the operating voltage is used to control thefirst oscillating transistor and the second oscillating transistor tooperate in a sub-threshold region.

Therefore, a crystal oscillator of an embodiment of the presentdisclosure may supply an oscillating circuit with a stable operatingvoltage and a stable operating current, so that power consumption of thecrystal oscillator is controllable, and the operating voltage enablestransistors to operate in a sub-threshold region, such that under a samecurrent condition, a transistor can provide larger transconductance,which is beneficial for fast starting oscillation of the crystaloscillator, and reducing the power consumption of the crystaloscillator.

With reference to the first aspect, in some implementation manners ofthe first aspect, where the second driving circuit includes a negativefeedback loop configured to receive the reference current generated bythe first driving circuit and generate the operating voltage accordingto the reference current.

With reference to the first aspect, in some implementation manners ofthe first aspect, the negative feedback loop includes a first currentcontrol transistor and a second current control transistor, and thefirst current control transistor and the second current controltransistor are configured to control the operating current of the firstoscillating transistor and the second oscillating transistor to be thestable current.

With reference to the first aspect, in some implementation manners ofthe first aspect, connection structures of the first current controltransistor and the second current control transistor is the same asconnection structures of the first oscillating transistor and the secondoscillating transistor, and a ratio of sizes of the first oscillatingtransistor and the first current control transistor is equal to a ratioof sizes of the second oscillating transistor and the second currentcontrol transistor.

With reference to the first aspect, in some implementation manners ofthe first aspect, the negative feedback loop further includes a thirdcurrent control transistor, an amplifier and a first voltage controltransistor, where the third current control transistor is configured tocontrol the operating current of the first oscillating transistor andthe second oscillating transistor, and the amplifier outputs theoperating voltage to the oscillating circuit through the first voltagecontrol transistor;

a drain of the third current control transistor is connected to thefirst driving circuit and is configured to receive the reference currentoutput by the first driving circuit, a gate of the first current controltransistor is connected to a drain of the first current controltransistor, a gate of the second current control transistor is connectedto a drain of the second current control transistor, and a gate of thethird current control transistor is connected to the drain of the thirdcurrent control transistor; and

the drain of the third current control transistor is further connectedto a first input end of the amplifier, a second input end of theamplifier is connected to the drain of the first current controltransistor and the drain of the second current control transistor, anoutput end of the amplifier is connected to a gate of the first voltagecontrol transistor, a source of the first current control transistor isconnected to a drain of the first voltage control transistor, and thedrain of the first voltage control transistor is configured to outputthe operating voltage.

With reference to the first aspect, in some implementation manners ofthe first aspect, the negative feedback loop further includes a fourthcurrent control transistor and a second voltage control transistor,where the fourth current control transistor is configured to control theoperating current of the first oscillating transistor and the secondoscillating transistor, the second voltage control transistor isconfigured to output the operating voltage to the oscillating circuit,and a drain of the fourth current control transistor is connected to thefirst driving circuit and is configured to receive the reference currentoutput by the first driving circuit;

the drain of the fourth current control transistor is further connectedto a gate of the second voltage control transistor, a gate of the fourthcurrent control transistor is connected to a gate of the first currentcontrol transistor and a gate of the second current control transistor,the gate of the first current control transistor is connected to a drainof the first current control transistor, and the gate of the secondcurrent control transistor is connected to a drain of the second currentcontrol transistor; and

a source of the first current control transistor is connected to asource of the second voltage control transistor, and the source of thesecond voltage control transistor is configured to output the operatingvoltage.

With reference to the first aspect, in some implementation manners ofthe first aspect, a first capacitor is further connected between asource of the first current control transistor and the ground, and thefirst capacitor is configured to perform phase compensation on theoperating voltage.

Optionally, the first capacitor may be configured to perform phasecompensation on the operating voltage and may also be configured toreduce a ripple of the operating voltage. In addition, the firstcapacitor is also configured to isolate a power supply of the firstdriving circuit and the operating voltage, which further improves apower supply rejection ratio PSRR, so that an influence of power supplynoise, interference, and the like on the crystal oscillator could bereduced.

With reference to the first aspect, in some implementation manners ofthe first aspect, a ratio of sizes of the first oscillating transistorand the first current control transistor is N:M, a ratio of sizes of thesecond oscillating transistor, the second current control transistor andthe third current control transistor is N:M:L, and the operating currentof the first oscillating transistor and the second oscillatingtransistor is I_(ref)N/L, where I_(ref) is the reference current.

With reference to the first aspect, in some implementation manners ofthe first aspect, the first driving circuit is further configured tocontrol an amplitude of an oscillating signal input to the crystal.

Since a source of the first oscillating transistor is configured toreceive an operating voltage, a variation of the operating voltage willeventually be converted into spurs or phase noise on an oscillatingsignal output by the crystal. In addition, if the operating voltagemakes the crystal oscillator enter a voltage limiting region, the spursor phase noise problem will be further deteriorated, however, employingthe first driving circuit of an embodiment of the present disclosure canmake the crystal oscillator operate at a proper amplitude, therebyavoiding a phase noise problem caused by the amplitude of the crystaloscillator entering the voltage limiting region and improving phasenoise performance of the crystal oscillator.

With reference to the first aspect, in some implementation manners ofthe first aspect, the first driving circuit includes a first biastransistor, a second bias transistor, a third bias transistor, a fourthbias transistor, a fifth bias transistor, a sixth bias transistor, and asecond capacitor;

where a drain of the first bias transistor is connected to a drain ofthe third bias transistor, the drain of the first bias transistor isconnected to a gate of the first bias transistor, a drain of the secondbias transistor is connected to a drain of the fourth bias transistor,the gate of the first bias transistor is connected to a gate of thesecond bias transistor, and a gate of the third bias transistor isconnected to the drain of the fourth bias transistor; a drain of thefifth bias transistor is connected to the drain of the fourth biastransistor, a source of the fifth bias transistor is connected to thegate of the fourth bias transistor, the gate of the fourth biastransistor is further connected to one end of the second capacitor, andthe other end of the second capacitor is configured to input theoscillating signal; and

a gate of the sixth bias transistor is connected to the gate of thesecond bias transistor, a source of the sixth bias transistor, a sourceof the first bias transistor and a source of the second bias transistorare connected, and a drain of the sixth bias transistor is configured tooutput the reference current.

With reference to the first aspect, in some implementation manners ofthe first aspect, when an AC signal in the oscillating signal increases,a drain voltage of the fourth bias transistor decreases, drain currentsof the first bias transistor, the second bias transistor, and the thirdbias transistor decrease, the reference current output by the firstdriving circuit decreases, the operating current of the firstoscillating transistor and the second oscillating transistor decreases,the transconductance of the first oscillating transistor and the secondoscillating transistor decreases, and an amplitude of the oscillatingsignal decreases.

With reference to the first aspect, in some implementation manners ofthe first aspect, in a case of an AC signal, the operating voltage isequivalent to a ground voltage, and the first oscillating transistor andthe second oscillating transistor provide the transconductance forstarting oscillation and maintaining oscillation of the crystal.

With reference to the first aspect, in some implementation manners ofthe first aspect, in the sub-threshold region, the transconductance ofthe first oscillating transistor and the second oscillating transistoris proportional to the operating current of the first oscillatingtransistor and the second oscillating transistor.

With reference to the first aspect, in some implementation manners ofthe first aspect, a gate of the first oscillating transistor isconnected to a gate of the second oscillating transistor, a drain of thefirst oscillating transistor is connected to a drain of the secondoscillating transistor, and a source of the first oscillating transistoris configured to receive the operating voltage.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic structural diagram of a typical crystaloscillator.

FIG. 2 is a schematic structural diagram of a crystal oscillatoraccording to an embodiment of the present disclosure.

FIG. 3 is an exemplary schematic structural diagram of an oscillatingcircuit in the crystal oscillator shown in FIG. 2.

FIG. 4 is an exemplary schematic structural diagram of a second drivingcircuit in the crystal oscillator shown in FIG. 2.

FIG. 5 is another exemplary schematic structural diagram of a seconddriving circuit in the crystal oscillator shown in FIG. 2.

FIG. 6 is an exemplary schematic structural diagram of an oscillatingcircuit in the crystal oscillator shown in FIG. 2.

FIG. 7 is another exemplary schematic structural diagram of the firstdriving circuit in the crystal oscillator shown in FIG. 2.

FIG. 8 is an exemplary schematic structural diagram of a crystaloscillator according to an embodiment of the present disclosure.

DESCRIPTION OF EMBODIMENTS

Embodiments of the present disclosure will be described hereinafter inconjunction with the drawings in the embodiments of the presentdisclosure.

FIG. 1 is a schematic diagram of a typical structure of a crystaloscillator. As shown in FIG. 1, a crystal oscillator 100 includes atransistor 110, a transistor 120, a crystal 130, a capacitor 140, and acapacitor 150.

A gate of the transistor 110 is connected to a gate of the transistor120, and a drain of the transistor 110 is connected to a drain of thetransistor 120, that is, the transistor 110 and the transistor 120 are aconnection structure of an inverter, and the transistor 110 and thetransistor 120 are configured to provide transconductance for startingoscillation and maintaining oscillation of the crystal 130.

However, the oscillator shown in FIG. 1 has the following problems:drain currents (or bias currents) of the transistor 110 and thetransistor 120 are determined by gate-source voltages V_(GS)S of thetransistors, while a sum of the V_(GS) of the transistor 110 and theV_(GS) of the transistor 120 is equal to a power supply voltage V_(cc).Since V_(cc) is affected by process, voltage, temperature, powerconsumption of the transistors is uncontrollable due to an influence ofPVT. Moreover, when V_(cc) is large, V_(GS)S of the transistors arenecessarily large, thus, under some process conditions it may cause aserious high-order effect of the transistors, for example, an verticalelectric field causes mobility to drop, and the problem is that althoughpower consumption of the transistors is large, the transistors in factdo not provide sufficient transconductance, which is not beneficial forfast starting oscillation of the crystal.

In view of this, an embodiment of the present disclosure provides anoscillating circuit, which can provide large transconductance forstarting oscillation and maintaining oscillation of an oscillator withlow power consumption.

FIG. 2 is a schematic structural diagram of a crystal oscillatoraccording to an embodiment of the present disclosure. As shown in FIG.2, a crystal oscillator 10 includes a crystal 200, an oscillatingcircuit 300, a first driving circuit 500 and a second driving circuit400, where:

the oscillating circuit 300 includes a first oscillating transistor anda second oscillating transistor, and the first oscillating transistorand the second oscillating transistor are configured to providetransconductance for starting oscillation and maintaining oscillation ofthe crystal 200;

the first driving circuit 500 is configured to generate a stablereference current, and output the reference current to the seconddriving circuit; and

the second driving circuit 400 is configured to supply an operatingvoltage to the oscillating circuit and make an operating current of thefirst oscillating transistor and the second oscillating transistor be astable current according to the reference current, where the operatingvoltage is used to control the first oscillating transistor and thesecond oscillating transistor to operate in a sub-threshold region.

Optionally, in the embodiment of the present disclosure, the oscillatingcircuit may be an oscillating circuit that adopts an inverter structure,for example, a Pierce crystal oscillating circuit, and therefore, thefirst oscillating transistor and the second oscillating transistor inthe oscillating circuit can both provide transconductance foroscillation of the crystal, which is benefit for rapidly startingoscillation of the crystal.

In the embodiment of the present disclosure, the second driving circuitis configured to receive the reference current output by the firstdriving circuit, supply an operating voltage to the oscillating circuit,and make the first oscillating transistor and the second oscillatingtransistor operate at a stable current. For example, the second drivingcircuit may control, according to an operating current control signal,the operating current of the first oscillating transistor and the secondoscillating transistor in the oscillating circuit to be the stablecurrent, where the operating voltage is a stable voltage that does notvary with PVT. Therefore, power consumption of the crystal oscillator ofthe embodiment of the present disclosure is controllable with respect toan existing crystal oscillator.

Optionally, the operating current control signal may be an operatingcurrent of the second driving circuit, for example, the operationcurrent of the second driving circuit is mirrored to the oscillatingcircuit through circuit structures of the second driving circuit and theoscillating circuit, and if the operation current of the second drivingcircuit is a stable current, the current mirrored to the oscillatingcircuit is also a stable current.

In addition, the operating voltage generated by the second drivingcircuit can enable the first oscillating transistor and the secondoscillating transistor to operate in a sub-threshold region. Sincetransconductance of the MOS transistor is proportional to a draincurrent of the MOS transistor in the sub-threshold region and conversionefficiency of current transconductance in the sub-threshold region ishigh, it means that a current required for same transconductance issmaller, which is beneficial for reducing the power consumption of thecrystal oscillator. Moreover, two oscillating transistors in theoscillating circuit of the embodiment of the present disclosure bothprovide transconductance, and therefore, the crystal oscillator of theembodiment of the present disclosure can provide larger transconductanceunder a same current condition, which is beneficial for fast startingoscillation and maintaining oscillation of the crystal oscillator.

It should be understood that the reference current generated by thefirst driving circuit is a stable current, and the stable currentreferred to herein does not mean that the reference current must be aconstant current, but only indicates that the reference current does notvary with PVT.

It should also be understood that the operating voltage output by thesecond driving circuit is a stable voltage, and the stable voltagereferred to herein does not mean that the operating voltage must be aconstant voltage, but only indicates that the operating voltage does notvary with PVT, or that the operating voltage varies with PVT but thesecond driving circuit may adjust the outputted operating voltage tocancel the influence brought by a variation of PVT.

In the embodiment of the present disclosure, the first driving circuitmay be configured to generate a stable reference current, for example,the first driving circuit may be implemented by adopting a typicalcircuit structure of a bias circuit, or may be implemented by adoptingother equivalent circuits, which is not limited to the embodiment of thepresent disclosure. Further, the first driving circuit may also beconfigured to control an amplitude of an oscillating signal input to thecrystal, which will be described in detail in the following embodiments.

Therefore, a crystal oscillator of an embodiment of the presentdisclosure may supply an oscillating circuit with a stable operatingvoltage and a stable operating current, so that power consumption of thecrystal oscillator is controllable, and the operating voltage can makethe transistor operate in a sub-threshold region, such that under a samecurrent condition, the transistor can provide larger transconductance,which is beneficial for fast starting oscillation of the crystaloscillator, and reducing the power consumption of the crystaloscillator.

In the prior art, as shown in FIG. 1, since one end of the transistor110 is controlled by an input voltage V_(CC), a variation of V_(CC) maybe converted into spurs or phase noise on the outputted oscillatingsignal V_(x0), which eventually leads to an oscillation amplitude of theoutputted oscillating signal V_(x0) uncontrollable, and if the inputvoltage V_(CC) makes the oscillator enter a voltage limiting region, theproblem of spurs and phase noise may become more serious.

Optionally, in the embodiment of the present disclosure, the oscillatingsignal output by the oscillating circuit 300 may be input to the firstdriving circuit 500, and the first driving circuit 500 may also controlthe amplitude of the oscillating signal, which can prevent theoscillating circuit from entering the voltage limiting region, and thusphase noise performance of the oscillating circuit could be improved.

Hereinafter, implementation manners of a crystal oscillator according toan embodiment of the present disclosure will be described in detail withreference to specific examples in FIGS. 3 to 8.

It should be understood that examples shown in FIGS. 3 to 8 are intendedto help those skilled in the art better understand embodiments of thepresent disclosure, rather than for limiting the scope of theembodiments of the present disclosure. It will be obvious for thoseskilled in the art to make various equivalent modifications orvariations according to FIGS. 3 to 8 as illustrated, which also fallwithin the scope of the embodiments of the present disclosure.

FIG. 3 is a schematic structural diagram of an oscillating circuitaccording to an embodiment of the present disclosure. As shown in FIG.3, an oscillating circuit 300 includes a first oscillating transistor301, a second oscillating transistor 302, a resistor 304, a capacitor305, and a capacitor 306.

Specifically, a drain of the first oscillating transistor 301 isconnected to a drain of the second oscillating transistor 302, a gate ofthe first oscillating transistor 301 is connected to a gate of thesecond oscillating transistor 302, a source of the first oscillatingtransistor 301 is configured to receive an operating voltage V_(dd), anoperating current of the drain of the first oscillating transistor 301is Id1, an operating current of the drain of the second oscillatingtransistor is Id2, the Id1 and the Id2 are a stable current and do notvary with PVT.

The gate of the first oscillating transistor 301 and the gate of thesecond oscillating transistor 302 are also connected to one end of theresistor 304, and the drain of the first oscillating transistor 301 andthe drain of the second oscillating transistor 302 are also connected tothe other end of the resistor 304.

In this embodiment, the resistor 304 is also connected in parallel witha crystal 303, and the crystal 303 may correspond to the crystal 200 inFIG. 2 that may include an input (in) end and an output (out) end. Oneend of the capacitor 306 is connected to an input end of the crystal303, the other end of the capacitor 306 is grounded, one end of thecapacitor 305 is connected to an output end of the crystal 303, and theother end of the capacitor 305 is grounded.

The resistor 304 is configured to supply DC bias voltages to the firstoscillating transistor 301 and the second oscillating transistor 302,and the capacitor 305, the capacitor 306 and the crystal 303 areconfigured to determine an oscillating frequency of the crystal, V_(xi)is an oscillating signal input to the crystal, and V_(x0) is anoscillating signal output from the crystal.

Optionally, in the embodiment of the present disclosure, in a case of asmall AC signal, the operating voltage V_(dd) is equivalent to a groundvoltage, and thus the first oscillating transistor 301 and the secondoscillating transistor 302 may both provide transconductance forstarting oscillation of the crystal 303, thereby enabling the crystal303 to start oscillation rapidly.

Optionally, in the embodiment of the present disclosure, the firstoscillating transistor 301 may be a PMOS transistor or other equivalentdevices, and the second oscillating transistor 302 may be an NMOStransistor or other equivalent devices, which is not limited in theembodiment of the present disclosure.

It should be understood that in the embodiment of the presentdisclosure, the crystal may also be referred to as a crystal plate, andthe crystal 303 may be a quartz crystal or a crystal of other materials,which is not limited by the embodiment of the present disclosure.

FIG. 4 is a schematic structural diagram of a second driving circuitaccording to an embodiment of the present disclosure. As shown in FIG.4, a second driving circuit 400 includes a negative feedback loop, wherethe negative feedback loop may include a first current controltransistor 411, a second current control transistor 412, a third currentcontrol transistor 413, an amplifier 430, and a first voltage controltransistor 422, where the first current control transistor 411, thesecond current control transistor 412, and the third current controltransistor 413 are configured to determine an operating current of afirst oscillating transistor and a second oscillating transistor.

A specific connection relationship is as follows: a drain of the thirdcurrent control transistor 413 is configured to receive a referencecurrent (corresponding to I_(ref) in FIG. 4) output by a first drivingcircuit, a gate of the first current control transistor 411 is connectedto a drain of the first current control transistor 411, a gate of thesecond current control transistor 412 is connected to a drain of thesecond current control transistor 412, and a gate of the third currentcontrol transistor 413 is connected to the drain of the third currentcontrol transistor 413; and the drain of the third current controltransistor 413 is connected to a first input end (such as an invertinginput end) of the amplifier 430, a second input end of the amplifier 430is connected to the drain of the first current control transistor 411and the drain of the second current control transistor 412, an outputend of the amplifier 430 is connected to a gate of the first voltagecontrol transistor 422, and a source of the first current controltransistor 411 is connected to a drain of the first voltage controltransistor 422.

The first voltage control transistor 422 is an executive element of theamplifier 430, that is, the amplifier 430 may implement control of theoperating voltage V_(dd) output to the oscillating circuit through thefirst voltage control transistor 422, and the drain of the first voltagecontrol transistor 422 is configured to output the operating voltageV_(dd) to the oscillating circuit.

In this embodiment, connection structures of the first current controltransistor 411 and the second current control transistor 412 are thesame as those of the first oscillating transistor and the secondoscillating transistor, that is, both are a connection structure of aninverter. Furthermore, a ratio of sizes of the first current controltransistor 411 and the first oscillating transistor 301 is equal to aratio (or a ratio of width to length, that is, a ratio of a gate widthto a gate length) of sizes of the second current control transistor 412and the second oscillating transistor 302. In this way, when the V_(GS)Sof the transistors are equal, the operating current of the firstoscillating transistor and the second oscillating transistor isproportional to the reference current, that is, the operating current isa stable current.

According to a virtual short principle of the input end of theamplifier, a voltage of the first input end of the amplifier is equal toa voltage of the second input end of the amplifier, that is,V_(a)=V_(b), that is, the first current control transistor 411, thesecond current control transistor 412 and the third current controltransistor 413 have equal gate voltages that are equal to V_(a) orV_(b), and the sources of the first current control transistor 411, thesecond current control transistor 412 and the third current controltransistor 413 are all grounded, and therefore, V_(GS)S of the firstcurrent control transistor 411, the second current control transistor412 and the third current control transistor 413 are equal, so that itcan be concluded that the drain current of the first current controltransistor 411, the drain current of the second current controltransistor 412 and the drain current of the third current controltransistor 413 are respectively proportional to a size of the firstcurrent control transistor 411, a size of the second current controltransistor 412 and a size of the third current control transistor 413.In other words, it is assumed that the sizes of the drain currents ofthe first current control transistor 411, the second current controltransistor 412 and the third current control transistor 413 are denotedas S1, S2, and S3, respectively, and the drain currents of the firstcurrent control transistor 411, the second current control transistor412, and the third current control transistor 413 are denoted as I1, I2,and I3, respectively, then I1/S1=I2/S2=I3/S3.

It can be seen in combination with FIGS. 3 and 4 that, when V_(xi) is aDC voltage, a connection structure of the second oscillating transistor302 is the same as that of the third current control transistor 413,that is, the V_(GS)S of those two are equal. Accordingly, the draincurrent of the second oscillating transistor 302 and the drain currentof the drain current of the third current control transistor 413 arerespectively proportional to the size of the second oscillatingtransistor 302 and the size of the third current control transistor 413.

In summary, if a ratio of sizes of the first oscillating transistor andthe first current control transistor is N:M, a ratio of sizes of thesecond oscillating transistor, the second current control transistor andthe third current control transistor is N:M:L, and the drain current ofthe third current control transistor is the reference current, denotedas I_(ref), then the drain current of the first current controltransistor 411 is I_(ref)·M/L, and the drain current (that is, theoperating current) of the first oscillating transistor 301 and thesecond oscillating transistor 302 is I_(ref)·N/L. Since the referencecurrent is a stable current that does not vary with PVT, the operatingcurrent of the first oscillating transistor and the second oscillatingtransistor is also a stable current that does not vary with PVT.

Optionally, in this embodiment, the second driving circuit 400 mayfurther include a first capacitor 440, which is connected between theoutput end of the operating voltage and the ground and is configured toperform phase compensation on the operating voltage to make theoperating voltage more stable.

Optionally, in this embodiment, the first current control transistor 411and the first voltage control transistor 422 may be PMOS transistors,and the second current control transistor 412 and the third currentcontrol transistor 413 may be NMOS transistors or other equivalentdevices, which is not limited in this embodiment of the presentdisclosure.

FIG. 5 is a schematic structural diagram of a second driving circuitaccording to another embodiment of the present disclosure. As shown inFIG. 5, a second driving circuit 400 may include a negative feedbackloop.

In this embodiment, the negative feedback loop may include a firstcurrent control transistor 414, a second current control transistor 415,a fourth current control transistor 416, and a second voltage controltransistor 424. Connection structures of the first current controltransistor 414 and the second current control transistor 415 are thesame as those of the first oscillating transistor and the secondoscillating transistor, that is, both are a connection structure of aninverter, and a ratio of sizes of the first current control transistor414 and the first oscillating transistor is equal to a ratio of sizes ofthe second current control transistor 415 and the second oscillatingtransistor, so that the operating current of the first oscillatingtransistor and the second oscillating transistor is proportional to thereference current, that is, the operating current is a stable current.

Specifically, a gate of the fourth current control transistor 416 isconnected to a gate of the first current control transistor 414 and agate of the second current control transistor 415, a drain of the fourthcurrent control transistor 416 is configured to receive a referencecurrent (corresponding to I_(ref) in FIG. 5) output by the first drivingcircuit, the drain of the fourth current control transistor 416 isconnected to a gate of the second voltage control transistor 424, and asource of the second voltage control transistor 424 is configured tooutput the operating voltage.

It should be understood that in this embodiment, the first currentcontrol transistor 414, the second current control transistor 415, thefourth current control transistor 416 and the second voltage controltransistor 424 are respectively similar to the first current controltransistor 411, the second current control transistor 412, the thirdcurrent control transistor 413 and the first voltage control transistor422 in the embodiment shown in FIG. 4, and functions of each of thetransistors will not be repeatedly described here.

It should also be understood that although an amplifier is not includedin this embodiment, actually, in this embodiment, the fourth currentcontrol transistor 416 functions as an amplifier, and the second voltagecontrol transistor 424 is equivalent to an executive element of theamplifier and is configured to control the operating voltage V_(dd)output by the second driving circuit 400.

Comparing circuit structures shown in FIG. 4 and FIG. 5, it can be seenthat the second driving circuit shown in FIG. 4 is a second-ordercircuit, therefore, it is required that the first capacitor 440 performsphase compensation on the outputted operating voltage V_(dd) to improvestability of the outputted operating voltage V_(dd); and the seconddriving circuit shown in FIG. 5 is a first-order circuit, it is notnecessary to perform phase compensation on V_(dd), but if the firstcapacitor 450 is provided in the second driving circuit shown in FIG. 5,a ripple of the operating voltage V_(dd) can be further reduced. Inaddition, the first capacitor 450 may also configured to isolate thepower supply V_(cc) of the first driving circuit and the operatingvoltage V_(dd), which further improves a power supply rejection ratio(PSRR), so that an influence of power supply noise, interference, andthe like on the crystal oscillator could be reduced, thereby furtherimproving the phase noise performance of the crystal oscillator.

Optionally, in this embodiment, the first current control transistor 414may be a PMOS transistor or other equivalent devices, and the secondvoltage control transistor 424, the second current control transistor415, and the fourth current control transistor 416 may be NMOStransistors or other equivalent devices, which is not limited in thisembodiment of the present disclosure.

FIG. 6 is a schematic structural diagram of a first driving circuitaccording to an embodiment of the present disclosure. As shown in FIG.6, a first driving circuit 500 may include a first bias transistor 511,a second bias transistor 512, a third bias transistor 513, a fourth biastransistor 514, and a sixth bias transistor 516.

A drain of the first bias transistor 511 is connected to a drain of thethird bias transistor 513, and a drain of the second bias transistor 512is connected to a drain of the fourth bias transistor 514;

a gate of the first bias transistor 511 is connected to a gate of thesecond bias transistor 512, and a gate of the third bias transistor 513is connected to a gate of the fourth bias transistor 514; and

the drain of the first bias transistor 511 is connected to the gate ofthe first bias transistor 511, the drain of the fourth bias transistor514 is connected to the gate of the fourth bias transistor 514, a gateof the sixth bias transistor 516 is connected to the gate of the secondbias transistor, a source of the sixth bias transistor 516, a source ofthe first bias transistor 511 and a source of the second bias transistor512 are connected, and a drain of the sixth bias transistor 516 isconfigured to output a reference current. For example, the drain of thesixth bias transistor 516 is connected to the drain of the third currentcontrol transistor 413 shown in FIG. 4 or to the drain of the fourthcurrent control transistor 416 shown in FIG. 5, so that the referencecurrent may be output to the drain of the third current controltransistor 413 or the drain of the fourth current control transistor416.

It can be seen from FIG. 6 that, the first driving circuit is a typicalbias circuit independent of a power supply voltage, therefore, the firstdriving circuit may output a stable reference current to the seconddriving circuit, and further, the second driving circuit may control anoperating current of the oscillating circuit to be a stable currentaccording to the stable reference current, and working principlesthereof will not be repeatedly described here.

FIG. 7 is a schematic structural diagram of a first driving circuitaccording to another embodiment of the present disclosure. The firstdriving circuit in this embodiment may be configured not only togenerate a reference current (corresponding to I_(ref) in FIG. 7), butalso to control an oscillation amplitude of a crystal oscillator.

As shown in FIG. 7, the first driving circuit may include a first biastransistor 521, a second bias transistor 522, a third bias transistor523, a fourth bias transistor 524, and a sixth bias transistor 526,which are respectively similar to the first bias transistor 511, thesecond bias transistor 512, the third bias transistor 513, the fourthbias transistor 514, and the sixth bias transistor 516 in the embodimentshown in FIG. 6, and working principles thereof will not be repeatedlydescribed here.

This embodiment differs from the embodiment shown in FIG. 6 in that: inthis embodiment, a fifth bias transistor 525 is connected in parallelbetween a gate of the fourth bias transistor 524 and a drain of thefourth bias transistor 524, and an oscillating signal V_(xi) that is anoscillating signal input to an input end of the crystal is input to thegate of the fourth bias transistor 524 through a second capacitor 530.

Thus, if V_(xi) is a DC signal, the gate of the fourth bias transistor524 and the drain of the fourth bias transistor 524 are shorted, and aworking manner of the fourth bias transistor 524 is the same as that ofthe fourth bias transistor 514 in the embodiment shown in FIG. 6.

If V_(xi) is an AC signal, the gate of the fourth bias transistor 524and the drain of the fourth bias transistor 524 are separated, and ACcoupling is performed on V_(xi) by the second capacitor 530, the ACsignal in V_(xi) may be extracted, that is, the AC signal in V_(xi) isapplied to the gate of the fourth bias transistor 524, while a DC biasvoltage generated by the fifth bias transistor 525 is also applied tothe gate of the fourth bias transistor 524.

Then, when the AC signal in the oscillating signal V_(xi) increases, adrain voltage (that is, V_(c) in FIG. 7) of the fourth bias transistor524 decreases, and drain currents of the first bias transistor 521, thesecond bias transistor 522, and the third bias transistor 523 decrease,so that the reference current output by the first driving circuit 500also decreases accordingly, meanwhile, the operating current of thefirst oscillating transistor and the second oscillating transistordecreases. Since the operating current of the first oscillatingtransistor and the second oscillating transistor is proportional totransconductance of the first oscillating transistor and the secondoscillating transistor, the transconductance of the first oscillatingtransistor and the second oscillating transistor decreases accordingly,and eventually an amplitude of the oscillating signal is decreased,thereby implementing control of an oscillation amplitude of the crystaloscillator.

Since a source of the first oscillating transistor is configured toreceive the operating voltage V_(dd), a variation of the operatingvoltage V_(dd) will eventually be converted into spurs or phase noise onthe oscillating signal output by the crystal. In addition, if V_(dd)makes the crystal oscillator enter a voltage limiting region, the spursor phase noise problem will be further deteriorated, however, employingthe first driving circuit of an embodiment of the present disclosure canmake the crystal oscillator operate at a proper amplitude, therebyavoiding a phase noise problem caused by the amplitude of the crystaloscillator entering the voltage limiting region and improving phasenoise performance of the crystal oscillator.

Optionally, in this embodiment, the first bias transistor 521, thesecond bias transistor 522, and the sixth bias transistor 526 may bePMOS transistors, or other equivalent devices, and the third biastransistor 523, the fourth bias transistor 524, and the fifth biastransistor 525 may be NMOS transistors, or other equivalent devices,which is not limited in this embodiment of the present disclosure.

FIG. 8 is an exemplary structural diagram of a crystal oscillatoraccording to an embodiment of the present disclosure. As shown in FIG.8, a crystal oscillator 600 includes a first driving circuit 601, asecond driving circuit 602, an oscillating circuit 603, and a crystal604, where the crystal 604 corresponds to the crystal 200 in FIG. 2 andthe crystal 303 in FIG. 3.

The oscillating circuit 603 includes a transistor M1 b, a transistor M2b, a capacitor C1, a capacitor C2 and a resistor R1, where thetransistor M1 b, the transistor M2 b, the capacitor C1, the capacitor C2and the resistor R1 respectively correspond to the first oscillatingtransistor 301, the second oscillating transistor 302, the capacitor306, the capacitor 305 and the resistor 304 in the embodiment shown inFIG. 3, and functions thereof are the same as functions of thecorresponding devices, and detailed operating processes will not berepeatedly described here.

The second driving circuit 602 includes a transistor M0 a, a transistorM1 a, a transistor M2 a, a transistor M2 c and an amplifier, whichrespectively correspond to the first voltage control transistor 422, thefirst current control transistor 411, the second current controltransistor 412, the third current control transistor 413 and theamplifier 430 in the embodiment shown in FIG. 4, and functions thereofare the same as functions of the corresponding devices, and detailedoperating processes will not be repeatedly described here.

The first driving circuit 601 includes a transistor M3, a transistor M4,a transistor M5, a transistor M6 and a transistor M7, which respectivelycorrespond to the sixth bias transistor 516, the first bias transistor511, the second bias transistor 512, the third bias transistor 513 andthe fourth bias transistor 514 in the embodiment shown in FIG. 6, andfunctions thereof are the same as functions of the correspondingdevices, and detailed operating processes will not be repeatedlydescribed here.

It should be understood that the circuit structure shown in FIG. 8 isonly an example, and the first driving circuit 601 may also beimplemented by using the circuit structure shown in FIG. 7, or using asimilar equivalent circuit; alternatively, the second driving circuit602 may be implemented by using the circuit structure shown in FIG. 5 ora similar equivalent circuit, which is not specifically limited in thisembodiment of the present disclosure.

The foregoing descriptions are merely specific embodiments of thepresent disclosure, but the protection scope of the present disclosureis not limited thereto, persons skilled in the art who are familiar withthe art could readily think of variations or substitutions within thetechnical scope disclosed by the present disclosure, and thesevariations or substitutions shall fall within the protection scope ofthe present disclosure. Therefore, the protection scope of the presentdisclosure shall be subject to the protection scope of the claims.

what is claimed is:
 1. A crystal oscillator, comprising: a crystal; anoscillating circuit comprising a first oscillating transistor and asecond oscillating transistor, wherein the first oscillating transistorand the second oscillating transistor are configured to providetransconductance for starting oscillation and maintaining oscillation ofthe crystal; a first driving circuit configured to generate a stablereference current; and a second driving circuit, configured to supply anoperating voltage to the oscillating circuit and make an operatingcurrent of the first oscillating transistor and the second oscillatingtransistor be a stable current according to the reference current,wherein the operating voltage is configured to control the firstoscillating transistor and the second oscillating transistor to operatein a sub-threshold region.
 2. The crystal oscillator according to claim1, wherein the second driving circuit comprises a negative feedback loopconfigured to receive the reference current generated by the firstdriving circuit and generate the operating voltage according to thereference current.
 3. The crystal oscillator according to claim 2,wherein the negative feedback loop comprises a first current controltransistor and a second current control transistor, and the firstcurrent control transistor and the second current control transistor areconfigured to control the operating current of the first oscillatingtransistor and the second oscillating transistor to be the stablecurrent.
 4. The crystal oscillator according to claim 3, whereinconnection structures of the first current control transistor and thesecond current control transistor is the same as connection structuresof the first oscillating transistor and the second oscillatingtransistor.
 5. The crystal oscillator according to claim 4, wherein aratio of sizes of the first oscillating transistor and the first currentcontrol transistor is equal to a ratio of sizes of the secondoscillating transistor and the second current control transistor.
 6. Thecrystal oscillator according to claim 3, wherein the negative feedbackloop further comprises a third current control transistor, an amplifierand a first voltage control transistor, wherein the third currentcontrol transistor is configured to control the operating current of thefirst oscillating transistor and the second oscillating transistor, andthe amplifier outputs the operating voltage to the oscillating circuitthrough the first voltage control transistor; a drain of the thirdcurrent control transistor is connected to the first driving circuit andis configured to receive the reference current output by the firstdriving circuit, a gate of the first current control transistor isconnected to a drain of the first current control transistor, a gate ofthe second current control transistor is connected to a drain of thesecond current control transistor, and a gate of the third currentcontrol transistor is connected to the drain of the third currentcontrol transistor; and the drain of the third current controltransistor is further connected to a first input end of the amplifier, asecond input end of the amplifier is connected to the drain of the firstcurrent control transistor and the drain of the second current controltransistor, an output end of the amplifier is connected to a gate of thefirst voltage control transistor, a source of the first current controltransistor is connected to a drain of the first voltage controltransistor, and the drain of the first voltage control transistor isconfigured to output the operating voltage.
 7. The crystal oscillatoraccording to claim 6, wherein a ratio of sizes of the first oscillatingtransistor and the first current control transistor is N:M, a ratio ofsizes of the second oscillating transistor, the second current controltransistor and the third current control transistor is N:M:L, and theoperating current of the first oscillating transistor and the secondoscillating transistor is I_(ref)N/L, wherein I_(ref) is the referencecurrent.
 8. The crystal oscillator according to claim 3, wherein thenegative feedback loop further comprises a fourth current controltransistor and a second voltage control transistor, wherein the fourthcurrent control transistor is configured to control the operatingcurrent of the first oscillating transistor and the second oscillatingtransistor, the second voltage control transistor is configured tooutput the operating voltage to the oscillating circuit, and a drain ofthe fourth current control transistor is connected to the first drivingcircuit and is configured to receive the reference current output by thefirst driving circuit; the drain of the fourth current controltransistor is further connected to a gate of the second voltage controltransistor, a gate of the fourth current control transistor is connectedto a gate of the first current control transistor and a gate of thesecond current control transistor, the gate of the first current controltransistor is connected to a drain of the first current controltransistor, and the gate of the second current control transistor isconnected to a drain of the second current control transistor; and asource of the first current control transistor is connected to a sourceof the second voltage control transistor, and the source of the secondvoltage control transistor is configured to output the operatingvoltage.
 9. The crystal oscillator according to claim 3, wherein a firstcapacitor is further connected between a source of the first currentcontrol transistor and the ground, and the first capacitor is configuredto perform phase compensation on the operating voltage.
 10. The crystaloscillator according to claim 1, wherein the first driving circuit isfurther configured to control an amplitude of an oscillating signalinput to the crystal.
 11. The crystal oscillator according to claim 10,wherein the first driving circuit comprises a first bias transistor, asecond bias transistor, a third bias transistor, a fourth biastransistor, a fifth bias transistor, a sixth bias transistor, and asecond capacitor; wherein a drain of the first bias transistor isconnected to a drain of the third bias transistor, the drain of thefirst bias transistor is connected to a gate of the first biastransistor, a drain of the second bias transistor is connected to adrain of the fourth bias transistor, the gate of the first biastransistor is connected to a gate of the second bias transistor, and agate of the third bias transistor is connected to the drain of thefourth bias transistor; a drain of the fifth bias transistor isconnected to the drain of the fourth bias transistor, a source of thefifth bias transistor is connected to the gate of the fourth biastransistor, the gate of the fourth bias transistor is further connectedto one end of the second capacitor, and the other end of the secondcapacitor is configured to input the oscillating signal; and a gate ofthe sixth bias transistor is connected to the gate of the second biastransistor, a source of the sixth bias transistor, a source of the firstbias transistor and a source of the second bias transistor are connectedto a power supply voltage, and a drain of the sixth bias transistor isconfigured to output the reference current.
 12. The crystal oscillatorof claim 11, wherein when an AC signal in the oscillating signalincreases, a drain voltage of the fourth bias transistor decreases,drain currents of the first bias transistor, the second bias transistor,and the third bias transistor decrease, the reference current output bythe first driving circuit decreases, the operating current of the firstoscillating transistor and the second oscillating transistor decreases,the transconductance of the first oscillating transistor and the secondoscillating transistor decreases, and an amplitude of the oscillatingsignal decreases.
 13. The crystal oscillator according to claim 1,wherein the first driving circuit comprises a first bias transistor, asecond bias transistor, a third bias transistor, a fourth biastransistor, a sixth bias transistor, and a second capacitor; wherein adrain of the first bias transistor is connected to a drain of the thirdbias transistor, and a drain of the second bias transistor is connectedto a drain of the fourth bias transistor; a gate of the first biastransistor is connected to a gate of the second bias transistor, and agate of the third bias transistor is connected to a gate of the fourthbias transistor; the drain of the first bias transistor is connected tothe gate of the first bias transistor, the drain of the fourth biastransistor is connected to the gate of the fourth bias transistor, agate of the sixth bias transistor is connected to the gate of the secondbias transistor; and a source of the sixth bias transistor, a source ofthe first bias transistor and a source of the second bias transistor areconnected to a power supply voltage, and a drain of the sixth biastransistor is configured to output the reference current.
 14. Thecrystal oscillator according to claim 1, wherein in a case of an ACsignal, the operating voltage is equivalent to a ground voltage, and thefirst oscillating transistor and the second oscillating transistorprovide the transconductance for oscillation and maintaining oscillationof the crystal.
 15. The crystal oscillator according to claim 1, whereinin the sub-threshold region, the transconductance of the firstoscillating transistor and the second oscillating transistor isproportional to the operating current of the first oscillatingtransistor and the second oscillating transistor.
 16. The crystaloscillator according to claim 1, wherein a gate of the first oscillatingtransistor is connected to a gate of the second oscillating transistor,a drain of the first oscillating transistor is connected to a drain ofthe second oscillating transistor, and a source of the first oscillatingtransistor is configured to receive the operating voltage.